Considering that NG is an image oriented forum (even though I would admit it is a nice space for casual chatting and serious discussions in the quasi-SNS style), the essential difference of between the UI for PCs and that for smartphones is the size of the screen. So, I think that viewing the forum on tablets is practically the same as viewing on the computer screen.
Then, I would doubt if the addition of the dedicated UI for the phone would be all that worth adding potential maintenance work of the administrators? That would make the transition of the server more complicated as well?
I do sometimes use phones to access NG, but that is only when I upload images shot with the phone, check out the new posts or make sure if my edit on the uncalibrated PC monitor is not drastically out of "standard". The UI is tiny, of course, but the links and buttons can be pinched to zoom in.